Cathode-ray tube and system to eliminate electrical discharges during indexing

ABSTRACT

In a beam-index type of cathode-ray tube for presenting a color image, in which a cathode-ray beam of high energy scans across a beam index structure located adjacent an insulating portion of the tube envelope, as for example in an odd half-harmonic color television index tube, it has been found that brief bursts of interference occur occasionally in the tube during use, which apparently are due to brief discharges in the tube near the front face, and which can upset the operation of the index control operation. The invention overcomes this difficulty by providing an electrically conductive coating in smooth intimate continuous contact with the exterior of the insulating portion of the tube adjacent at least one edge of the scanned area of the tube. When one or more starting index stripes are utilized along one edge of the scanned area, the coating is placed on the exterior of the tube close to this edge. The coating is maintained at a substantially fixed potential, usually ground potential.

llite States Patent Sunstein ]March 20, 1973 [5 CATHODE-RAY TUBE ANDSYSTEM TO ELIMINATE ELECTRICAL DISCHARGES DURING INDEXING [76] Inventor:David E. Sunstein, 464

Conshohocken State Road, Bala- Cynwyd, Pa. 19004 [22 Filed: Jan. 8, 1971[21] Appl.No.: 105,047

Primary ExaminerCarl D. Quarforth Assistant Examiner-J. M. PotenzaAttorney-Howson and Howson [5 7] ABSTRACT In a beam-index type ofcathode-ray tube for presenting a color image, in which a cathode-raybeam of high energy scans across a beam index structure located adjacentan insulating portion of the tube envelope, as for example in an oddhalf-harmonic color television index tube, it has been found that briefbursts of interference occur occasionally in the tube during use, whichapparently are due to brief discharges in the tube near the front face,and which can upset the operation of the index control operation. Theinvention overcomes this difficulty by providing an electricallyconductive coating in smooth intimate continuous contact with theexterior of the insulating portion of the tube adjacent at least oneedge of the scanned area of the tube. When one or more starting indexstripes are utilized along one edge of the scanned area, the coating isplaced on the exterior of the tube close to this edge. The coating ismaintained at a substantially fixed potential, usually ground potential.

14 Claims, 5 Drawing Figures CATHODlE-IRAY TUBE AND SYSTEM TO ELIMINATEELECTRICAL DISCHARGES DURING INDEXING BACKGROUND OF THE INVENTION Avariety of cathode-ray tube devices are known in which a cathode-raybeam is accelerated to impinge with high energy against surfaces in thetube, and is scanned across an arc in the tube with beam indexingelements disposed in spaced-apart predetermined locations so that, bydetecting the times at which the beam impinges each index element,information in the form of an electrical signal can be derived whichcontains information as to the position of the beam at successive timesas it scans across the index elements.

One specific type of cathode-ray tube device with particular referenceto which the invention will be described is a color television beamindex tube such as is shown and described in my U.S. Pat. No. 3,013,113,issued Dec. 12, 1961; 2,892,123, issued June 23, 1959 and 3,305,788,issued Feb. 21, 1967. In such a tube the image presentation meanstypically comprises a repeating sequence of colored light producingelements in the form of triplets of vertical stripes at the front faceof the cathode-ray tube, the different stripes in each triplet producinglight of different colors (e.g. red, green, blue) when impinged by thebeam. The intensity of the cathode-ray beam as it traverses individualstripes in each triplet is so controlled as to produce such relativeamounts of the different colors emitted by adjacent stripes as toproduce, in the eye of the viewer, the color desired for thecorresponding portion of the color image. The intensity of the beam iscontrolled by a received color television signal, and in order toproduce a proper color image the information in the received signalmust, in effect, be synchronized with impingement of the beam uponparticular ones of the stripes in each triplet. Usually the side of theimage forming stripes which faces the source of the cathoderay beam iscovered with a thin light-reflective metallic layer pervious to thecathode-ray beam, and index elements are placed upon the rear, or inner,side of the metallic layer in predetermined geometric positions acrossthe tube, in a specific predetermined relation to the positions of thestripes which produce the differently colored light. The metallic layertends to maintain the beam-facing side of the image-presentation meansat a nearly vuniform potential, and to reflect the image light forwardlytoward the viewer. In arrangements such as those referred to in myabove-cited patents, these so-called running index elements may comprisephotoemissive stripes which are scanned by the beam at a rate which is anon-integral multiple (greater than 1) times the rate at which thetriplets are scanned by the beam. Each of the running stripes produces aburst of electromagnetic radiation (light) each time the beam impingesit, and an appropriate photoresponsive device, such as a photocell,viewing the interior side of the index structure, will respond to thesebursts of electromagnetic radiation to produce a correspondingelectrical index signal.

By the above-mentioned non-integral relationship, contamination of theindex signal due to intensitymodulations of the scanning beam at therate at which the triplets are scanned ("triplet frequency") isminimized. However, the running index signal is itself ambiguous in thatthe successive peaks of the index signal do not occur at the time ofscanning of the same stripes of different triplets, and merelyconverting the index signal frequency to the triplet frequency byconventional means will not resolve this ambiguity. My above-citedpatents describe a method and apparatus for correcting the ambiguousrunning index signal to produce an unambiguous index signal at tripletfrequency, by means of at least one so-called starting index stripes solocated as to be traversed by the scanning beam prior to the horizontalscanning of each line in the viewing area of the image-presentationdevice, thereby to produce an unambiguous indication of the position ofthe beam. When more than one starting stripe is used, the horizontalposition of the starting stripes is made such that they are scanned bythe beam at a rate which is a subharmonic (including 1) of the tripletfrequency. The one or more starting stripes are typically positionedalong one marginal edge of the image-presentation means at the front ofthe cathoderay tube, or even along a side interior surface of the tubeadjacent the edge of the viewing area where they will still be scannedby the beam at the start of each horizontal scan.

Typically the metallic layer extends to, and makes contact with, thesecond-anode coating on the interior of the sides of the cathode-raytube. The metallic layer therefore tends to maintain both the viewingarea and the area occupied by the starting stripes at somewhere near thesame high potential as the second anode, and as mentioned above this isone of its intended purposes. In addition, it is common to employ ametal mounting ring around the outside of the cathode-ray tube near thefront face thereof, which commonly is connected to a substantially fixedsource of potential highly negative with respect to the second anodepotential, for example chassis ground potential, or the potential of thepositive or negative low-voltage supply.

1 have found that when such a beam index type of cathode-ray tube isutilized to produce indexing signals for beam control purposes, fromtime to time there will commonly occur small bursts of interferencegenerated by or at the cathode-ray tube which upset the normal operationof the index signal generating and control system. This interference iscommonly such that, in a color image-presentation cathode-ray tube ofthe beam indexing type, the color image will momentarily become greatlydistorted along a given scanning line or lines at the time of or shortlyafter each such burst. The occurrences of such interference burstscausing erroneous color of scan lines are generally coincident withaudible or visual tricks, or small sparkings, apparently at or on thecathode-ray tube near the front thereof, and are generally mostprominent and objectionable in the displayed image when the beamintensity is strongly modulated, or upon change from scenes of onebrightness to scenes of different brightness.

Accordingly it is an object of the present invention to provide new anduseful cathode-ray tube apparatus of the beam index type.

A further object is to provide such apparatus in which the harmfuleffects of short bursts of interference which tend to be generatedwithin or at the cathode-ray tube near the cathode-ray tube screen aregreatly reduced or entirely eliminated.

Another object is to provide an improved construction of colortelevision image-presentation tube of the beam index type.

A further object is to provide such a construction which accomplishessuch improvement and is simple to construct and operate.

SUMMARY OF THE INVENTION These and other objects of the invention areachieved, in a cathode-ray tube of the beam indexing type in which indexsignals are generated by scanning the beam across the index elements andin which at least parts of some of the index elements are positionedadjacent an insulating portion of the cathode-ray tube, by the provisionof a layer of electrically-conductive material extending over and inintimate contact with the exterior surface of the insulating envelopeportion adjacent the positions within the tube where said parts of saidindex element are located, together with means for maintaining saidlayer at a substantially fixed potential.

Preferably the layer of electrically-conductive material is an adherentcoating on the exterior surface of the cathode-ray tube envelope, andthe means for maintaining the layer at a substantially fixed potentialpreferably comprises a source of such substantially fixed potentialelectrically connected to said layer, the fixed potential preferablybeing chassis ground potential. In a preferred form, a metal bandextends around the cathode-ray tube, over and against the layer orcoating, and in turn is connected to the source of fixed potential.

Where the cathode-ray tube is of the type containing image presentationmeans having a predetermined viewing area at or adjacent the front ofthe cathode-ray tube, the layer or coating will normally extend alongthe outside of the cathode-ray tube envelope adjacent one edge of theviewing area, so as to provide its desired electrical effects withoutinterfering with normal viewing.

More particularly, where the cathode-ray tube is of the type comprisingcolor television image presentation means made up of groups or tripletsof segments producing light of different colors, the external coating orlayer will be located on the outside of the tube adjacent that margin ofthe normal viewing area at which scanning of horizontal lines begins,particularly where the beam indexing system is of the type utilizing oneor more starting stripes located adjacent said margin.

In the preferred form of the invention, there is em ployed a colortelevision image-presentation tube of the type comprising triplets ofnominally vertical stripes of differently colored light producingsegments arranged across the front of the cathode-ray tube so as to bescanned transversely at a predetermined triplet rate by the cathode-raybeam, running index segments disposed on the rear or inward side of ametallic layer covering the light-producing segments and arranged inspaced-apart positions so as to produce a signal when scanned by thecathode-ray beam having a frequency which is an odd half-harmonic of thefrequency of scanning of the color triplets, and at least one startingindex segment located adjacent the margin at which horizontal scanningof the image begins and positioned such that when it is scanned by thebeam it produces a signal which can resolve any phase ambiguityotherwise created by the odd-half harmonic index stripe structure,together with the improvement comprising an adherent coating ofelectrically-conductive material extending along the exterior of thecathode-ray tube near the front face thereof and adjacent the startingindex segment or segments, the coating being connected to a referencepotential differing substantially from that supplied to the metalliclayer. In one preferred form, the metallic layer is internally connectedto the second anode of the cathode-ray tube, and the exteriorelectrically-conductive coating extends around the front edge of thetube to a position immediately adjacent that edge of the normal viewingarea at which horizontal scanning thereof begins, the coating preferablybeing of a light-absorbant material so that it not only provides thedesired inhibition of the bursts of interference mentioned above, butalso aids in preventing transmission of image light through the wall ofthe cathode-ray tube envelope and around the metallic layer to withinthe tube where it might interfere with photo-detection of theimpingement of the beam upon index segments.

By utilizing the electrically-conductive layer or coating in accordancewith the invention, it has been found that the bursts of interference,which normally would occur occasionally and interfere with the indexingsignal generation and control, are greatly reduced or entirelyeliminated.

BRIEF DESCRIPTION OF FIGURES Other objects and features of the inventionwill be more readily understood from a consideration of the followingdetailed description, taken in connection with the accompanying drawingsin which:

FIG. 1 is a schematic diagram principally in block form, showing in planview a cathode-ray tube constructed in accordance with the invention,and illustrating one system in connection with which the improvement ofthe invention may be employed;

FIG. 2 is an enlarged plan view of the cathode-ray tube shown in FIG. 1,showing furtherdetails thereof;

FIG. 3 is a front elevational view of the cathode-ray tube shown in FIG.2;

FIG. 4 is a fragmentary sectional view taken along lines 4-4 of FIG. 2;and

FIG. 5 is an enlarged fragmentary view, in section, showing a comer anda portion of the front construction of the cathode-ray tube of FIG. 2.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Referring now to theembodiment of the invention specifically shown in the drawings by way ofexample only, FIG. 1 illustrates one type of color television receiverto which the invention is applicable. An appropriate antenna It)delivers color television radiofrequency signals to the RF, firstdetector, IF amplifier, second detector, and processing circuits 12which derive and supply signals over line 14 to audio circuits 16 andthence to loudspeaker 18 to produce the desired reproduction of theaudio portion of the received transmissions. The circuits 12 also supplyoutput signals over line 20 to video circuits 22, wherein they areappropriately processed and combined with or controlled by index signalssupplied from index circuits 24 over line 26. The output signals fromvideo circuits 22 are supplied over line 28 to the intensity-controllinggrid 30 of the color-image reproducing cathode-ray tube 32 to controlthe intensity of the scanning cathode-ray beam therein.

Appropriate output signals from video circuits 22 are also supplied overline 36 to control the deflection and control circuits 38, thereby toproduce on output line 40 thereof deflection signals appropriate forapplication to the deflection control means 42, which may be aconventional horizontal and vertical deflection yoke surrounding theneck of the cathode-ray tube 32. Circuits 38 also provide suitableoutput signals over line 44 to control generation, in the high-voltagecircuits 46, of a suitable second-anode high voltage which is deliveredover high-voltage lead 48 to the secondanode connection 50 of thecathode-ray tube 32.

As shown in FIGS. 1, 2, 3 and 5, the cathode-ray tube 32 is encircled,near its front face, by a flexible metal band 54, which overlies andmakes electrical contact with, an electrically-conductive layer orcoating 56 on the exterior of cathode-ray tube 32. Band 54 includes asuitable screw-type clamping arrangement 58 for drawing it tightlyaround the exterior of the cathode-ray tube, and in this example isprovided with a plurality of mounting flanges such as 60 for securing itto a vertical support, and with an additional pair of flanges 62 bywhich it may be affixed to, and in electrical contact with, the metallicchassis 64 (chassis ground). The latter mounting and electricalconnection may be provided by means of the screws such as 66 (FIG. 5)extending through appropriate slotted holes such as 68 in thechassis-mounting flanges.

The usual low-voltage supply circuits 69 provide the requiredlow-voltage power for operating the various elements of the system.

In the present example the invention will be described with particularreference to its use in connection with photo-index tubes of the typedescribed in detail in my above-cited patents. Accordingly, in this casethere is shown a photocell 70 mounted within a light-sealed enclosure 72having one end open, or translucent, so as to view the interior side ofthe front of the cathode-ray tube 32 by way of a window 74 in the coneof the cathode-ray tube; the window may comprise translucent glass ofthe cathode-ray tube envelope, the interior side of which is free of thenormally-opaque second anode coating 76. While for simplicity a simplephotocell is shown, in practice a photomultiplier tube is preferred.Light from the photoresponsive index elements to be describedhereinafter is received by the photocell 70 and converted into anelectrical index signal supplied over the line 80 (FIG. 1) to the indexcircuits 24, wherein it is appropriately processed into a form suitablefor supply to the video circuits 22. v

The purpose, construction and operation of such indexing circuits, andthe manner in which the index signals produced thereby may be combinedwith received television signals, are described in my abovecitedpatents, and certain aspects thereof are claimed therein. Accordinglythe details of their construction and operation will not be repeatedhere. In brief, the

photocell 70 picks up short bursts of electromagnetic radiation from theindex stripes which radiation may be in the visual portion of thespectrum but will be referred to herein as light even if not in thevisible spectrum. These bursts of light are produced by the impingementof the cathode-ray beam, during its scanning, upon certain startingstripes and certain running stripes which will be described herein withparticular reference to FIG. 5 and are fully described in theabove-cited patents. In general, the running stripes extend across theimage-presentation means throughout the viewing area, and are soarranged as to produce an ambiguous index signal substantially free fromcontamination due to modulation of the intensity of the cathode-ray beamat the triplet frequency by the received color-television information.The starting stripes produce a signal prior to the scanning of theportion of each horizontal line lying within the viewing area, and arearranged adjacent that edge of the viewing area at which horizontalscanning of the viewing area begins. The signal generated by thesestarting stripes is used in the index circuits 24 to produce, from theambiguous index signals, corresponding unambiguous index signalsthroughout the scanning of the remainder of each horizontal line.

Referring now particularly to FIGS. 3 and 5, in this example at leastthe front face 84 and the adjacent side surface 86 of the cathode-raytube 32 are made of an electrically insulating material such astransparent glass. Typically the front portion 84 of the tube and theimmediately adjacent side portions may be made of one piece of glass,and the remainder of the cone portion 86 and the neck of the cathode-raytube made as a separate portion, the two portions later beingappropriately secured together by a glass weld' 88, by well-knowntechniques. This makes it easier to fabricate and apply the variousimage-presentation and indexing elements on the interior surface of thefront of the cathode-ray tube.

The normal color television image viewing area 90 (FIG. 3) comprises aplurality of groups or triplets, such as T and T of stripe-like elementssuch as those labeled R 0,, 8,, R 0,, B and which are responsive toimpingement by the cathode-ray beam to produce a different color oflight for each element or stripe within a given triplet; for example,red, green, and blue light. These groups of elements or stripes coverthe entire viewing area of the cathode-ray tube, and in this example areprovided by known techniques directly on the interior surface of theglass front 84 of the cathode-ray tube.

On the interior side of the cathode-ray tube, extending over thephosphor stripe elements and around the edge of the front of the tubeonto the interior side surfaces thereof, there is provided aconventional evaporated metallic film 94, which is typically ofaluminum. The second-anode coating 76, which is typically of aquadag,preferably extends over, and in contact with a portion of the metalliclayer 94, as shown in FIG. 5.

On the rear, or beam, side of the metallic layer 94 are located therunning index stripes such as 95, 96, 97 and 98, for example, which arespaced across the tube in the direction of horizontal beam scanning sothat the rate of their scanning by the beam is 3/2 times the rate atwhich the triplets T and T are scanned by the beam. In other words,there are three indexing stripes for each two triplets. in the exampleshown, these index stripes are placed in alignment with the spacesbetween alternate color stripes, as shown. These index stripes aredesignated as the running index stripes since they extend entirelyacross the scanned viewing area of the image-presentation device, incontrast with the starting index stripes such as 99, 100 and 101, whichare placed along a marginal edge (to the left in FIG. of the normalviewing area so as to be scanned successively by the cathode-ray beam atthe start of each horizontal line scanning, prior to the scanning of therunning index stripes in the viewing area.

As described in my above-cited patents, the starting stripes 99, 100 and101 are spaced laterally apart from each other in such manner that therate at which they are scanned by the cathode-ray beam is a subharmonicof the rate at which the triplets of phosphor stripes are scanned by thebeam. Both the running stripes and the starting stripes in this exampleare photo-responsive, so as to produce momentary bursts of light whenthey are impinged by the cathode-ray beam, although secondary-electronemissive index stripes may be used. Preferably the material used for theindexing stripes is a short-persistence phosphor so that the bursts oflight which each index stripe produces on impingement will disappearquickly after each scanning thereof.

The photo-responsive device 70, which views the interior of the front ofthe cathode-ray tube, responds to the bursts of light from the startingstripes and from the running stripes to produce a correspondingelectrical index signal, which is then processed in the index circuits24 of FIG. 1 and applied to the video circuits 22 to produce the desiredcontrol of the intensity modulation of the beam, so as to produce anaccurate reproduction of the color image on the image-presentationdevice.

When the type of cathode-ray tube described above is provided with asurrounding supporting metallic band, such as band 54, and this band isconnected to a source of potential differing substantially from thesecond anode potential, for example is connected to chassis ground, theabove-described small discharges, tickings, and bursts of interferencehave been found to occur. While all of the reasons for these bursts ofinterference and the mechanisms of their production are not entirelyclear, they tend to occur more often when the beam is modulated or whena scene changes, and when they occur, they cause miscoloring or lack ofimage in a few scan lines, momentarily.

It is believed that a significant factor is that the band 54, by itsphysical nature cannot readily make an intimate continuous electricalcontact with the exterior of the glass envelope of the cathode-ray tube,but at best makes contact primarily over a number of discrete relativelysmall areas; within each such area even minute variations in the surfaceof the tube or of the band will produce small gaps between band and tubesurface. A voltage charge can then build up on the exterior of theglass, either by electrical leakage through the glass or by electronspenetrating into the interior of the glass, or otherwise. When thischarge reaches the breakdown potential of the surrounding air or of thesurface film on the glass, it can discharge quickly. This discharge canproduce light to which the photocell 70 will respond, undesirably; or itcan produce undesirable changes of potential within the cathode-raytube, particularly in that the thin metallic layer 94, in practice, maynot always provide a definite reliable low-resistance connection betweenthe second anode and all portions of the front part of the cathode-raytube supposedly covered by the layer. Accordingly, this metallic layermay not actually maintain all parts of the cathode-ray tube adjacent theindexing stripes at a single fixed potential, particularly when theseindexing stripes are scanned by the modulated cathode-ray beam. Whateverthe reasons, the above-described intermittent and occasional bursts ofinterference, which can upset the indexing operation, have beenexperienced, particularly when using the grounded support band 54 indirect contact with the exterior of the insulating cathode-ray tubeenvelope.

However subtle is the cause of these very annoying visual effects in thefinal image, their occurrence is overcome by use of theelectrically-conductive layer or coating 56, which in this exampleextends around the left-hand half of the periphery of the viewing area,on the exterior of the cathode-ray tube envelope, in the form of a bandof electrically-conductive material. It is positioned on the exterior ofthose portions of the cathode-ray tube wall whose inner surfaces arenearest the starting stripes 99, and 101, and as shown preferablyextends around the front edge of the cathode-ray tube onto a portion ofthe front surface, as far as the edge of the normal viewing area. Thiscauses the coating 56 to be disposed adjacent not only starting stripessuch as 99, 100 which are on the side or at the corner of thecathode-ray tube, but also adjacent starting stripes such as 101 whichare disposed along the front of the cathode-ray tube.

A suitable material for coating 56 is graphite in the form known asaquadag, although other electricallyconductive materials may beutilized. It is conveniently applied by painting a liquid suspension,solution or slurry of the conductive material onto the exterior of thecathode-ray tube, and allowing it to dry. The coating is maintained at afixed potential in this example by means of the metal mounting band 54,which is secured to the conductive chassis 64.

With this arrangement, even though the contact between the band 54 andthe layer 56 may not be intimate or continuous, it is sufficient toprovide adequate electrical connection to the coating 56, which in turnextends continuously over the surface of the envelope in stable intimatecontact therewith, adjacent the starting stripes. It is believed thatthe principal mechanisms by which the coating 56 reduces the undesiredbursts of interference are by elimination of charge buildup on theexterior of the bulb to the point where electrical discharge in air oralong the glass surface can take place; and/or by providing uniformcapacitive coupling between the metallic layer 94 and the grounded band54. Thus the metallic layer 94 and the coating 56 on the inner and outersides of the wall of the insulating glass envelope comprise a capacitorsuch that any substantial tendencies toward rapid changes in thepotential of the metallic layer 94 (or portions thereof, if it is brokeninto segments insulated from each other) caused for example by scanningof starting stripes by the cathode-ray beam, are in effect capacitivelyshorted to ground. In this way local high-intensity sudden changes inpotential are prevented, and the bursts of interference which normallyresult therefrom eliminated or greatly reduced.

As mentioned above, the coating is preferably of light-absorbantmaterial, such as aquadag for example, so that the light generated bythe image phosphors such as R 6,, B which might tend to be internallyreflected around the adjacent corner of the cathode-ray tube, along theinterior of the tube wall, and into the photocell, tends to be absorbedat the interface between the coating 56 and the exterior of the tubeenvelope and thus is reduced in intensity.

While the invention has been described with particular regard tospecific embodiments thereof in the interest of complete definiteness,it will be understood that it may be embodied in a variety of formsdiverse from those specifically shown and described, without departingfrom the scope and spirit of the invention as defined by the appendedclaims.

What is claimed is: v

1. In a cathode-ray tube indexing system comprising a cathode-ray tubethe envelope of which includes an electrically insulating portion, meansfor generating a cathode-ray beam in said tube, high-voltage means insaid tube for accelerating said beam toward the inner surface of saidenvelope portion, index elements in said tube adjacent predeterminedareas of said inner surface of said envelope portion and responsive toimpingement by said beam to generate detectable index emanations, meansresponsive to deflection signals for deflecting said beam to cause it totraverse said index elements, and means responsive to said emanations toproduce electrical index signals indicative of the times at which saidindex elements are impinged by said beam:

the improvement which comprises a layer of electrically-conductivematerial extending over and in intimate contact with the exteriorsurface of said envelope portion adjacent said areas of said innersurface, and means for maintaining said layer at a substantially fixedpotential.

2. Apparatus in accordance with claim 1, in which said layer comprisesan adherent coating of electrically-conductive material on said exteriorsurface.

3. Apparatus in accordance with claim 1, in which said means formaintaining said layer at a substantially fixed potential comprises anelectrical connection to said layer and a source of substantially fixedpotential connected to said connection.

4. Apparatus in accordance with claim 3, in which said connectioncomprises a metal band extending around said tube and over and againstsaid coating.

5. Apparatus in accordance with claim 1, in which said envelope containsimage-presentation means at the front of said tube for producing animage in response to impingement by said beam when it scans across saidimage-presentation means from one side to the other thereof, and inwhich said layer extends along the exterior of said tube adjacent saidone side of said image-presentation means.

6. Apparatus in accordance with claim 5, in which at least some of saidindex elements extend outside the normal viewing area of saidimage-presentation means and said layeris disposed on the outer surfaceof said envelope adjacent said at least some index elements.

7. Apparatus in accordance with claim 1, in which said tube containscolor image-presentation means at the front thereof comprising animage-forming area made up of a plurality of different phosphor elementsdifferently responsive to impingement by said beam to produce light ofdiffering colors, said deflecting means being effective, when operated,to scan said imageforrning area from one side to the other thereof withsaid beam, said index elements being located at predetermined positionswith respect to said phosphor elements including positions adjacent saidone side, said layer extending over a part of said insulating portion ofsaid tube adjacent said one side.

8. Apparatus in accordance with claim 1, in which said tube comprises atransparent front face having on the inner surface thereof a layer ofphosphor elements arranged in a regular order and differently responsiveto impingement by said beam to produce light of differing colorsthroughout a normal viewing area, and a metallic layer pervious to saidbeam and disposed over said phosphor layer on the side from which saidbeam approaches said phosphor layer, at least some of said indexelements being located adjacent but beyond one edge of said viewing areaand on said metallic layer, said layer of electrically conductivematerial extending along the side surface of said tube adjacent saidedge.

9. In a cathodeqay indexing tube comprising a set of laterallyspaced-apart running index stripes in said tube positioned to beimpinged sucessively with a first periodicity by the cathode-ray tubebeam as it is deflected tranversely to said stripes to produce anambiguous index signal representative of the position of said beam withrespect to said running stripes, and at least one starting stripe alongone edge of said set in predetermined laterally-spaced relation to saidset of running index stripes for producing another index signalindicative of the position of said beam with respect to a particular oneof said running stripes, the improvement comprising:

a layer of electrically-conductive material in intimate contact with theexterior of said tube in a position adjacent said at least one startingstripe.

10. In the tube of claim 9, means for maintaining said layer at asubstantially fixed potential.

11. In a color television image-presentation tube comprising acathode-ray tube envelope, means for forming a cathode-ray beam in saidtube, a transparent front face on said tube, color-image forming meanson said front face and comprising an array ofperiodicallypositionedgroups of light-emissive stripes, differentstripes in each group being differently responsive to impingement bysaid beam to produce light of differing colors, means for scanning saidbeam transversely to said stripes at successively different verticalpositions to produce a television raster on said color-image formingmeans, means responsive to color television signals and to index signalsfor controlling the intensity of said beamas it scans the stripes ineach of said groups to produce a color of light from each successivelyscanned group area which is appropriate to the desired color of thetelevision image in that area, a conducting layer extending over saidimage-forming means on the side of approach by said beam, a plurality ofrunning photoindex stripes periodically recurrent in the horizontaldirection along said image-forming means on the side of said conductivelayer impinged by said beam and recurrent transversely of saidimage-forming means with a periodicity different from that of saidlight-emissive stripes, starting photo-index stripes positioned on saidside of said conductive layer and adjacent the edge of saidimage-forming means first impinged by said beam in its horizontalscannings, said starting stripes having a different periodicity alongsaid horizontal direction than said running stripes, and meansresponsive to light emanations from said starting stripes and from saidrunning stripes to generate an index signal representative of theinstantaneous horizontal position of said beam, the improvementcomprising:

an intimate continuous electrically-conductive coating on the exteriorof said tube adjacent said edge of said image-forming means.

12. In a beam-indexing color-image display cathoderay tube having abeam-indexing structure therein responsive to impingement by the beam asit performs successive horizontal line-scanning of said structure toproduce index signals, the improvement which comprises means formaintaining at a substantially fixed potential the exterior portions ofsaid tube adjacent the regions of said structure at which saidline-scannings begin.

13. The tube of claim 12, in which said structure includes a portionalong one edge thereof at which said scannings being for producingunambiguous indications of impingement by said beam, and said means formaintaining said exterior portions of said tube at a substantially fixedpotential comprises a conductive coating overlying said exteriorportions and means for maintaining said coating at a substantially fixedpotential.

14. The tube of claim 13, in which said tube is of the type employinggroups of generally-vertical imageforming line segments differentlyresponsive to impingement by said beam to produce light of differentcolors extending throughout a normal viewing area and said beam-indexingstructure comprises an array of beam responsive index line segmentsdisposed inwardly of said tube from said image-forming line segments andin predetermined geometric relation thereto, said array including indexline segments positioned along said edge.

1. In a cathode-ray tube indexing system comprising a cathoderay tubethe envelope of which includes an electrically insulating portion, meansfor generating a cathode-ray beam in said tube, high-voltage means insaid tube for accelerating said beam toward the inner surface of saidenvelope portion, index elements in said tube adjacent predeterminedareas of said inner surface of said envelope portion and responsive toimpingement by said beam to generate detectable index emanations, meansresponsive to deflection signals for deflecting said beam to cause it totraverse said index elements, and means responsive to said emanations toproduce electrical index signals indicative of the times at which saidindex elements are impinged by said beam: the improvement whichcomprises a layer of electricallyconductive material extending over andin intimate contact with the exterior surface of said envelope portionadjacent said areas of said inner surface, and means for maintainingsaid layer at a substantially fixed potential.
 2. Apparatus inaccordance with claim 1, in which said layer comprises an adherentcoating of electrically-conductive material on said exterior surface. 3.Apparatus in accordance with claim 1, in which said means formaintaining said layer at a substantially fixed potential comprises anelectrical connection to said layer and a source of substantially fixedpotential connected to said connection.
 4. Apparatus in accordance withclaim 3, in which said connection comprises a metal band extendingaround said tube and over and against said coating.
 5. Apparatus inaccordance with claim 1, in which said envelope containsimage-presentation means at the front of said tube for producing animage in response to impingement by said beam when it scans across saidimage-presentation means from one side to the other thereof, and inwhich said layer extends along the exterior of said tube adjacent saidone side of said image-presentation means.
 6. Apparatus in accordancewith claim 5, in which at least some of said index elements extendoutside the normal viewing area of said image-presentation means andsaid layer is disposed on the outer surface of said envelope adjacentsaid at least some index elements.
 7. Apparatus in accordance with claim1, in which said tube contains color image-presentation means at thefront thereof comprising an image-forming area made up of a plurality ofdifferent phosphor elements differently responsive to impingement bysaid beam to produce light of differing colors, said deflecting meansbeing effective, when operated, to scan said image-forming aRea from oneside to the other thereof with said beam, said index elements beinglocated at predetermined positions with respect to said phosphorelements including positions adjacent said one side, said layerextending over a part of said insulating portion of said tube adjacentsaid one side.
 8. Apparatus in accordance with claim 1, in which saidtube comprises a transparent front face having on the inner surfacethereof a layer of phosphor elements arranged in a regular order anddifferently responsive to impingement by said beam to produce light ofdiffering colors throughout a normal viewing area, and a metallic layerpervious to said beam and disposed over said phosphor layer on the sidefrom which said beam approaches said phosphor layer, at least some ofsaid index elements being located adjacent but beyond one edge of saidviewing area and on said metallic layer, said layer of electricallyconductive material extending along the side surface of said tubeadjacent said edge.
 9. In a cathode-ray indexing tube comprising a setof laterally spaced-apart running index stripes in said tube positionedto be impinged sucessively with a first periodicity by the cathode-raytube beam as it is deflected tranversely to said stripes to produce anambiguous index signal representative of the position of said beam withrespect to said running stripes, and at least one starting stripe alongone edge of said set in predetermined laterally-spaced relation to saidset of running index stripes for producing another index signalindicative of the position of said beam with respect to a particular oneof said running stripes, the improvement comprising: a layer ofelectrically-conductive material in intimate contact with the exteriorof said tube in a position adjacent said at least one starting stripe.10. In the tube of claim 9, means for maintaining said layer at asubstantially fixed potential.
 11. In a color televisionimage-presentation tube comprising a cathode-ray tube envelope, meansfor forming a cathode-ray beam in said tube, a transparent front face onsaid tube, color-image forming means on said front face and comprisingan array of periodically-positioned groups of light-emissive stripes,different stripes in each group being differently responsive toimpingement by said beam to produce light of differing colors, means forscanning said beam transversely to said stripes at successivelydifferent vertical positions to produce a television raster on saidcolor-image forming means, means responsive to color television signalsand to index signals for controlling the intensity of said beam as itscans the stripes in each of said groups to produce a color of lightfrom each successively scanned group area which is appropriate to thedesired color of the television image in that area, a conducting layerextending over said image-forming means on the side of approach by saidbeam, a plurality of running photo-index stripes periodically recurrentin the horizontal direction along said image-forming means on the sideof said conductive layer impinged by said beam and recurrenttransversely of said image-forming means with a periodicity differentfrom that of said light-emissive stripes, starting photo-index stripespositioned on said side of said conductive layer and adjacent the edgeof said image-forming means first impinged by said beam in itshorizontal scannings, said starting stripes having a differentperiodicity along said horizontal direction than said running stripes,and means responsive to light emanations from said starting stripes andfrom said running stripes to generate an index signal representative ofthe instantaneous horizontal position of said beam, the improvementcomprising: an intimate continuous electrically-conductive coating onthe exterior of said tube adjacent said edge of said image-formingmeans.
 12. In a beam-indexing color-image display cathode-ray tubehaving a beam-indexing structure therein responsive to impingement bythe beam as it performs successive horizontal line-scanning of saidstructure to produce index signals, the improvement which comprisesmeans for maintaining at a substantially fixed potential the exteriorportions of said tube adjacent the regions of said structure at whichsaid line-scannings begin.
 13. The tube of claim 12, in which saidstructure includes a portion along one edge thereof at which saidscannings being for producing unambiguous indications of impingement bysaid beam, and said means for maintaining said exterior portions of saidtube at a substantially fixed potential comprises a conductive coatingoverlying said exterior portions and means for maintaining said coatingat a substantially fixed potential.
 14. The tube of claim 13, in whichsaid tube is of the type employing groups of generally-verticalimage-forming line segments differently responsive to impingement bysaid beam to produce light of different colors extending throughout anormal viewing area and said beam-indexing structure comprises an arrayof beam responsive index line segments disposed inwardly of said tubefrom said image-forming line segments and in predetermined geometricrelation thereto, said array including index line segments positionedalong said edge.